1. Field of the Invention
The present invention relates to a rotor for a permanent magnet type rotating machine, which operates as an induction machine at the machine's starting and also a synchronizing machine at the rated driving.
2. Description of the Related Art
Generally, the permanent magnet-reluctance type rotating machine comprises a stator having armature windings and a rotor rotating inside the stator. In the machine, the rotor is provided with no windings forming a field system but with permanent magnets disposed on side faces of respective projecting portions of a rotor core having irregularities on its outer periphery. Therefore, the machine is characterized in that the structure is simple in comparison with the conventional wound-rotor type rotating machine.
Between the rotor core and the stator, this permanent magnet-reluctance type rotating machine has a small magnetic reluctance about each projecting portion and a large magnetic reluctance about each recessed portion due to the structural unevenness on the rotor core. In this way, there is a difference between the projecting portion and the recessed portion in terms of magnetic energy stored at a clearance (air gap) between the rotor core and the stator due to the current flowing through the armature windings, so that the output is mainly produced by the change in magnetic energy.
Note, the unevenness about the rotor is not necessarily provided by its geometrical structure and, in short, the "magnetic" unevenness may be formed about the rotor. Therefore, the rotating machine may be provided with a constitution by which the magnetic reluctance and flux can be changed dependently of the rotational position of the rotor.
Note, the reluctance type rotating machine using no permanent magnet has a problem of power reduction when the current flowing the armature windings increases since the leakage flux into the magnetically recessed portions, i.e. the interpole portions, is increased by the enlargement of magnetically saturated area in the projecting portions, i.e. the pole portions.
The permanent magnet-reluctance type rotating machine is capable of avoiding the deterioration of output since the permanent magnets on respective pole sides of the rotor core do act as the magnetic reluctance to reduce the leakage flux directing to the interpole portions.
In addition to the above-mentioned function to reduce the leakage flux, the permanent magnets have an effect to generate the reluctance torque due to the interaction between their own flux and the flux from the armature windings.
In this way, the permanent magnet-reluctance type rotating machine is constructed so as to exhibit different magnetic reluctance dependent on the rotational position of the rotor due to the magnetic unevenness about the peripheral face of the rotor core and also ensures the power output since the leakage flux for the interpole portions is reduced by the permanent magnets on the pole sides.
In the permanent magnet-reluctance type rotating machine, however, there is a problem of difficult self-starting because the self-retaining torque is increased by the permanent magnets on the pole sides.
In order to ensure the self-starting torque, there have been attempted many countermeasures, for example, to adopt an inverter as supplementary starting means, to overlap the rotor with an additional starting cage, etc. However, these countermeasures cause the structure of the rotating machine to be complicated overall. Especially in case of the self-starting cage, there is also raised a problem of increasing the magnetic reluctance to reduce the main magnetic flux.
Under the situation, the applicant of the present invention has previously proposed a rotor for the permanent magnet-reluctance type rotating machine which is capable of improving its starting characteristics without requiring any supplemental starting means (Japanese Patent Application No. 10-275797). In this machine, for example, the starting cage is made of magnetic material and starting conductors are provided in the form of a "deep-groove".
At the machine's starting, this permanent magnet-reluctance type rotating machine makes the starting conductors on the periphery of the rotor produce an induction torque corresponding to the change of armature current, thereby allowing the rotor to operate as the induction machine. At the machine's rated driving, the permanent magnet-reluctance type rotating machine allows the rotor to operate as the synchronous machine by both of the reluctance torque originated in the magnetic unevenness on the rotor surface and the torque based on the interaction of the flux of the permanent magnets on the pole sides with the flux from the armature windings.
As to the above-mentioned "self-starting" permanent magnet-reluctance type rotating machine, it is known that when the flux from the permanent magnets in the rotor core, namely, "magnet flux" gets large or the difference in magnetic reluctance between the projecting portions and the recessed portions of the rotor core gets large, the machine is apt to be started with difficulty.
It is also known that, conversely, if the magnet flux or reluctance is small, then the operative conversion is apt to be difficult at the machine's pull-in as one turning point from the starting operation as the induction machine to the normal operation as the synchronous machine.
In order to carry out the machine's starting and pull-in without a hitch, it is necessary to establish the number of magnet flux and the value of reluctance appropriately.
Despite the fact that the general tendencies of magnet flux and reluctance upon the machine's starting and pull-in operations has been recognized as mentioned above, there has not been provided any definite establishment against the magnet flux and the reluctance yet.
Therefore, there presently exists a situation where, for example, one permanent magnet-reluctance type is easy to start but hard for pull-in, while the other rotating machine is easy for its pull-in but hard to start.